Necromancy requires summoning the spirit of the deceased, and to bind them, whether it is a ghost, a zombie or Sue, they require similar power, skill and technique to a summoning and binding. That evening Harry summoned and bound the Erl King and Sue, difficult to say which is the greater feat, but we know Sue impressed the Erl King to the extent that he forgave Harry’s past transgressions.
Body swapping again falls into the same skill set, it is taking the spirit from a living body and severing the link, as Corpsetaker was able to do to Butters as a spirit, but not to Mort. Then using you will to take over the vacant vessel.
Whilst summoning is something wizards are taught, necromancy is not, but the next nearest use of the skill set for summoning a spirit are those for summoning an entity. Demonreach has multiple times been described as a spirit, being a physical manifestation of the the island, a genius loci, literally “spirit of the place”. In summoning Molly we have the whole debate between spirit and the flesh in escaping the summoning circle. Harry summoned and bound Demonreach by accident, doing a different ritual or Sanctum invocation to allow him to draw on local power.
Corpsetaker was looking for a body with talent, Molly was too powerful for her to overcome as a spirit especially with Lea in tow, Mort was the next most powerful practitioner in Chicago, more powerful than suspected. I suspect both Harry and Mort are both exceptionally strong in spirit, even if their implementation is very different, the difference is that Harry is as strong in other areas like fire. Mort would be able to summon entities like Harry can and bind them, he probably could have been a warden of Demonreach.
Corpsetaker would also be strong in spirit so fitting into a new body whether by possession or swap is easiest if one is strong in spirit, especially if that is all that you have left to work with. It makes it easier to carry over non-spirit capabilities into a new body. Luccio was strong but perhaps not as strong as an ectomancer or Necromancer in spirit, she could be forcibly swapped by Corpsetaker and be unable to use her full capabilities in her new body, unlike Corpsetaker or Harry.
If strength in spirit rests with the spirit and earth, wind, fire and water with the body this would explain everything, why Harry couldn’t manipulate the real world as a ghost, why Luccio has problems adjusting and Corpsetaker did not, why Luccio was vulnerable to Peabody’s influence etc. As a wizard ages they become more resistant to mental influence and develope foreknowledge, is this the spirit acting on the body, why Luccio hopes to get some power back? How Charity allowed her power to waste away and why Mort was able to regain his?
Necromancy, summoning and body swap ALL require the ability in spirit to dominate another spirit, and bind it, in a circle, to zombie flesh or mortal plane with a drum, or to a living body.